The function zfs_nicenum that converts number to human-readable output
uses a index to a string of letters. This patch limits the index to
the length of the string.
Signed-off-by: Christer Ekholm <che@chrekh.se>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes#3122
8bdcfb5396